A conventional drill is disclosed, for example, in Japanese utility model publication No. 56-41954. The prior art drill includes a cutting edge at its tip, a tapered portion extending from the cutting edge, and a rectilinear portion extending from the tapered portion. Abrasive grains are applied to the outer surface of each of the portions. The drill is fabricated to provide a plurality of axially extending spiral grooves. Also, a plurality of grooves are formed in the outer periphery of the rectilinear portion and extend between the spiral grooves. This drill also serves as a honing tool. Japanese utility model publication No. 55-23768 also discloses a drill in which grains made of a material harder than at least the drill body, such as diamond and cubic boron nitride, are deposited to its tip where removed materials are likely to clog. Another tool, shown in FIG. 7 and commonly referred to as a core drill, includes a hollow cylindrical shank. The shank has a cylindrical front end to which highly abrasive grains are applied. The abrasive grains are bonded by metal.
No attempts have been made to eliminate the occurrence of chippage or chattering, particularly, at the entrance of each hole. When the existing drills are used to drill holes in a material made of glass or ceramics, then the material must be chamferred to remove chippage. With specific reference to FIG. 6, a twist drill (D) has super abrasive grains thereon. When a sheet of glass (Wg) is drilled by the twist drill (D), chippage 1 and chattering 2 are likely to occur at the entrance and exit of each hole, respectively.
As shown in FIG. 7 a conventional core drill (T) is hollow and cylindrical in shape. Super abrasive grains are applied to the front end of the core drill to provide a cutter (3). When this core drill (T) is used to drill a ceramic plate (Ws), chippage 1 and chattering 2 are also likely to occur at the entrance and exit of each hole, respectively. The core drill tends to create more serious chippage than the twist drill (D) with a chisel edge 4 at its tip shown in FIG. 6. In addition, the chattering 2 results from cracks 5 extending outwardly from the front end of the core drill (T).
In order to overcome the foregoing problems, a plurality of tools were used to first drill one side of a workpiece and then the other side in an aligned fashion. Another attempt was made to attach a dummy material to the rear surface of a workpiece by means of an adhesive and remove the dummy material from the workpiece after drilling is completed. However, these attempts all result in a substantial decrease in the productivity and are not economical.